The present invention relates generally to optoelectronic devices, and specifically to high-speed shutters for image and optical data modulation.
Optoelectronic shutters are well known in the art. Such shutters open and shut in response to an electrical waveform or pulse applied thereto, generally without moving mechanical parts. They are used, inter alia, in high-speed image capture applications, for which mechanical shutters are typically too slow. Optoelectronic shutters known in the art include liquid crystal shutters, electrooptical crystal shutters and gated image intensifiers.
Liquid crystal shutters are simple and inexpensive to manufacture. Their speed, however, is inherently limited to about 20 xcexcsec switching time. Moreover, in their open state, liquid crystal shutters typically transmit only about 40% of the light incident thereon, whereas in their closed state, they still transmit at least 0.1% of the incident light.
Electrooptical crystal shutters can be switched quickly, on the order of 0.1 nanosecond. They require a collimated light input, however, and have only a narrow acceptance angle within which they can shutter incident light efficiently. The crystals themselves are expensive, and costly, high-speed, high-voltage electronics are also needed to switch the shutters on and off at the rated speed.
Image intensifiers generally comprise an electron tube or microchannel plate, with a photoelectric photocathode input and a light-emitting phosphor-coated anode at the output. Gated intensifiers further include high-speed switching circuitry, which enables them to be gated on and off quickly, with typical switching times as fast as 1 nanosecond. For light to be effectively shuttered or amplified by the intensifier, it must be focused on the photocathode. Although intensifiers are manufactured in large quantities, the manufacturing process involves metal-to-glass vacuum sealing, which is complex, labor intensive and therefore costly. Partly as a result of this complexity, gated intensifiers tend to be large compared to their active area and are available in a very limited range of shapes and sizes.
It is an object of the present invention to provide a compact, high-speed, solid-state optoelectronic shutter, which may be manufactured at relatively low cost in large quantities.
In some aspects of the present invention, the shutter is used for modulating light that is received by an image capture device, such as a high-speed CCD camera.
In other aspects of the present invention, the shutter is used in modulating an image or an array of optically-encoded data, for example, in the framework of a system for optical data processing.
It is a further object of the present invention to provide a method for manufacturing the shutter.
In preferred embodiments of the present invention, an optoelectronic shutter comprises a generally planar substrate made of semiconductor material, having mutually substantially parallel input and output surfaces. A planar photodiode layer is formed on the input surface of the substrate, and a planar light-emitting diode (LED) layer is formed on the output surface, opposite the photodiode layer. A gate layer is formed intermediate the photodiode and LED layers, preferably adjacent the photodiode layer. Preferably, transparent, electrically conductive coatings, for example, indium tin oxide (ITO), are applied to at least a portion of each of the input and output surfaces. An additional biasing layer is preferably formed intermediate the gate and LED layers, for back-biasing the LED.
When light strikes the photodiode layer, photoelectrons are created. Ordinarily, when there is no voltage or only a relatively small voltage applied between the input and output surfaces, the electrons remain in the photodiode layer and recombine, as they are unable to pass the gate. Under these conditions, the shutter is closed.
To open the shutter, a control voltage, preferably in the range of 5 to 15 volts, is applied between the surfaces, to bias the LED positively with respect to the photodiode. In some embodiments of the invention, higher or lower voltages may be used. Preferably the voltage is applied to the conductive coating on the surfaces. This voltage creates a potential difference across the substrate, between the photodiode and the LED. In this state, photoelectrons that are produced in the photodiode pass through the gate and substrate to the LED layer, which emits light in response to the incident photoelectrons. This process continues until the control voltage is removed, whereupon the shutter closes.
Preferably, the substrate comprises silicon, GaAs, InP or other semiconductor material known in the art, preferably in the range of 0.5 to 2 mm thick and 1 to 40 mm across. More preferably, the substrate comprises a high-electron mobility, substantially single crystal of one of the above-mentioned materials, wherein the crystal is oriented so that one of the crystal axes is substantially perpendicular to the input and output surfaces. In this way, when the control voltage is on, photoelectrons emitted by the photodiode travel ballistically along the crystal axis perpendicular to the surfaces, generally without substantial scattering and without significant photoelectron divergence in directions other than perpendicular to the surface. Hence, a photon striking at any point on the input surface of the shutter and generating a photoelectron in the photodiode layer there will cause a photon to be emitted by the LED layer at a corresponding point on the output surface. As a result, if an image is focused onto the input surface, it will be reproduced at the output surface with minimal blurring or distortion.
The active aperture of the shutter, defined by the areas of the photodiode and LED, may be as large as 40 mm across and may be made circular, square or rectangular, depending on the application. Thus, shutters in accordance with the present invention are more compact and may have a substantially greater ratio of active aperture to thickness than high-speed shutters known in the art, such as gated intensifiers and electrooptical crystal shutters.
In some preferred embodiments of the present invention, the photodiode layer comprises an avalanche photodiode. Preferably, an additional transparent conductive layer is interposed between the photodiode layer and the gate, and a voltage preferably of between 20 and 100 volts is applied to this conductive layer so as to reverse-bias the avalanche diode. Each photon incident on the input surface that is absorbed by the photodiode layer will cause an xe2x80x9cavalanchexe2x80x9d of electrons, to be generated, as is known in the art. When the control voltage is applied, these electrons pass through the gate to the LED layer. Thus, shutters according to these preferred embodiments transmit images with enhanced efficiency and can even provide a modicum of image intensification.
In preferred embodiments of the present invention, the shutter is produced using methods of semiconductor device fabrication known in the art. After the substrate has been suitably cut and polished, the gate, photodiode and LED layers are preferably formed thereon by means of epitaxy, MOCVD and/or ion implantation. Electrical leads are then bonded to appropriate locations on the shutter, specifically to the input and output surfaces thereof, and the shutter is suitably packaged for its application.
It will be appreciated that shutters may be mass-produced in accordance with the principles of the present invention at substantially lower cost than high-speed shutters known in the art. Shutters in accordance with preferred embodiments of the present invention generally include only a single, solid-state component, largely comprising low-cost, readily-available materials. Fabrication of such shutters may be substantially automated. Shutters in accordance with preferred embodiments of the present invention may be made to operate at relatively low voltage: typically 5-10 volts, or at most 100 volts when an avalanche photodiode layer is used. Gated intensifiers known in the art generally require high voltage, typically at least 6,000 volts.
In some preferred embodiments of the present invention, a shutter as described above is used to modulate light input to an image capture device, such as a CCD camera. For example, the shutter may be used in image capture devices substantially as described in PCT patent application number PCT/IL97/00084, entitled xe2x80x9cOptical Shutter,xe2x80x9d filed Mar. 7, 1997, in the Israel receiving office, and particularly in camera systems for range-gated and three-dimensional distance-responsive imaging, as described in PCT patent applications PCT/IL96/100020, PCT/IL96/00021 and PCT/IL96/00025, all filed Jun. 20, 1996. All of these PCT patent applications are assigned to the assignee of the present patent application, and their disclosures are incorporated herein by reference. Light passing through the shutter may be focused onto an image detector, such as a CCD array. Alternatively, the shutter may be directly coupled to the image detector, for example, by attaching the shutter to a fiber-optic faceplate that is coupled to the detector, by fastening the shutter directly to the image detector surface or by using a relay lens.
In other preferred embodiments of the present invention, the shutter is used as a part of a system for optical computing. The shutter is used to switch or modulate simultaneously a matrix of optically-encoded data bits or an electronic image.
There is therefore provided, in accordance with a preferred embodiment of the invention, a solid-state optoelectronic shutter, comprising: a semiconductor material, having formed therein or thereon: a planar photodiode, having a planar surface, and optically communicating with the input; a planar LED layer, having a planar surface substantially parallel to the planar photodiode, and optically communicating with the output; and a planar gate layer, intermediate the planar photodiode and the planar LED.
Preferably, the substrate comprises substantially a single crystal, and wherein an axis of the crystal is oriented in a direction substantially perpendicular to the planar surfaces of the photodiode and the LED.
Preferably, the semiconductor material comprises a material selected from the group consisting of silicon, GaAs and InP.
In a preferred embodiment of the invention, the photodiode has a planar PIN structure. Alternatively, the photodiode comprises a planar avalanche photodiode. Preferably the shutter comprises a transparent, conductive layer between the photodiode and the gate layer, wherein an electrical potential is applied to the conductive layer to reverse-bias the photodiode.
Preferably, a control voltage is applied across the gate layer so as to permit electrons to flow therethrough, from the photodiode to the LED, thereby opening the shutter.
In a preferred embodiment of the invention, the semiconductor material is comprised in a generally planar substrate having input and output faces. Preferably, the shutter comprises first and second transparent, conductive coatings on the input and output faces, respectively, wherein the control voltage is applied between the first and second coatings. Preferably the shutter further includes metal coatings on peripheral portions of the input and output faces, wherein the metal coatings are electrically coupled to the transparent, conductive coatings for application of the control voltage therethrough.
In a preferred embodiment of the invention, the planar photodiode is proximate the input face. Alternatively or additionally, the planar LED is proximate the output face.
In a preferred embodiment of the invention the planar photodiode, the gate layer and the planar LED are formed proximate one of the input and output faces. Preferably, the substrate is thinned between the formed planar devices and the other face.
In a preferred embodiment of the invention at least one of the faces is a face of semiconductor material epitaxially grown on the substrate and wherein at least one of the planar photodiode and the planar LED is formed in the epitaxially grown material.
There is further provided, in accordance with a preferred embodiment of the invention, an optical shutter comprising a photodiode which receives light an produces charge carriers; an LED which receives the charge carriers and produces light responsive to the carriers; and a gate which gates the flow of carriers from the photodiode to the LED. Preferably the shutter includes a semiconductor filled region, situated intermediate the photodiode and the LED, through which the charge carriers flow.
In a preferred embodiment of the invention, the semiconductor material comprises a material selected from the group consisting of silicon, GaAs and InP.
In a preferred embodiment of the invention, the photodiode has a PIN structure. Alternatively, the photodiode comprises an avalanche photodiode. Preferably the shutter comprises a transparent, conductive layer between the photodiode and the gate layer, wherein an electrical potential is applied to the conductive layer to reverse-bias the photodiode.
Preferably, a control voltage is applied across the gate layer so as to permit electrons to flow therethrough, from the photodiode to the LED, thereby opening the shutter.
There is further provided, in accordance with a preferred embodiment of the invention, a method for producing an optoelectronic device, comprising: providing a generally planar substrate made of semiconductor material, having input and output faces; producing a gate layer within the substrate between the input and output faces; producing a planar photodiode proximate the input face, external to the gate layer; and producing a LED proximate the output face, external to the gate layer.
Preferably, the method comprises depositing transparent, conductive coatings on the input and output faces. Preferably the method further includes depositing metal coatings on peripheral portions of the input and output faces, in electrical contact with the transparent, conductive coatings thereon. Preferably, the method further includes attaching electrical leads to the metal coatings, for applying a control voltage thereto.
In a preferred embodiment of the invention producing the gate layer comprises implanting ions in the substrate. Alternatively or additionally, producing the photodiode comprises doping the substrate to form a planar PIN structure therein. Alternatively or additionally producing the photodiode comprises forming an avalanche photodiode structure at the input face of the substrate. Preferably, the method includes producing a conductive layer intermediate the gate layer and the photodiode for applying a biasing voltage to the photodiode.
In a preferred embodiment of the invention, the photodiode is formed proximate the input face. Alternatively or additionally, the LED is formed proximate the output face.
In a preferred embodiment of the invention, the photodiode, the gate layer and the LED are formed proximate one of the input and output faces. Preferably, the method includes thinning the substrate between the formed photodiode, gate layer and LED and the other of the input and output faces.
There is further provided, in accordance with a preferred embodiment of the invention, an optoelectronic shutter produced according to the method described above.
There is farther provided, in accordance with a preferred embodiment of the invention, an integrated image detection device, comprising: an optical detector array, having a front surface; and a shutter as described above, fixed to the front surface of the array so as to modulate light incident on the array through the shutter. Preferably, the output surface of the shutter is directly attached to the front surface of the array. Alternatively, the device comprises a faceplate, which conveys an optical image between first and second sides thereof, wherein the first side of the faceplate is attached to the output face of the shutter, and the second side of the faceplate is attached to the front surface of the array.
There is further provided, in accordance with a preferred embodiment of the invention, a method for producing an integrated imaging device, comprising providing an optoelectronic shutter as described above and fixing the shutter to a front surface of an optical detector array. Preferably, fixing the shutter to the front surface of the detector array comprises cementing the shutter to the surface. Alternatively, fixing the shutter to the front surface of the detector array comprises cementing a first side of a faceplate to the surface and cementing the shutter to a second side of the faceplate, opposite the first side and optically communicating therewith.